Anatomy of the Human Osseous Spiral Lamina and Cochlear Partition Bridge: Relevance for Cochlear Partition Motion

Volume rendering technique and high-resolution microCT: 3D exploration of the cochlear anatomy

PURPOSE

Given its unique anatomical position and the amalgamation of bony and soft tissues within the cochlea, exploring its intricacies poses persistent challenges. Histopathology remains the gold standard in research, but given its inherent limitations, there is a clear need for innovative alternatives. The integration of microCT technology with advanced volume rendering techniques emerges as a promising approach for overcoming the hurdles associated with anatomical investigations of the cochlea.

METHODS

We seamlessly integrated high-resolution microCT cochlear images with medical imaging analysis software to create detailed 3D anatomical images of the human cochlea without the need of sample processing.

RESULTS

Volume rendering allowed a multiplanar, non-destructive, detailed anatomical evaluation of the human cochlea, including its capillary system, as well as soft tissue visualization at single-micron resolution in 3D.

CONCLUSION

The use of volume rendering in cochlear anatomical studies is underexplored despite the prevalence of 3D reconstruction. This technique presents a promising avenue for scientific investigation, providing researchers with unprecedented insights that can potentially benefit patients with hearing disorders.

Finite element analysis of the osseous spiral lamina's influence on inner ear fluid flow during bone conduction stimulation

Recent studies have investigated the anatomy and motion of the human cochlear partition, revealing insights into the flexible nature of the osseous spiral lamina (OSL). These investigations have primarily focused on air-conducted stimulation, leaving the impact of the OSL's flexibility during bone-conducted (BC) stimulation largely unexplored. By considering the OSL as either flexible or rigid in a finite element model of the human inner ear, we examined the effect of the OSL's flexibility on the fluid flow in the inner ear during BC stimulation, which was divided into contributors entering via the oval window (OW) and rigid body stimulation. Our results with rigid body stimulation indicate that the OSL facilitates an increased differential fluid flow at the round window compared to the OW, aligning with experimental observations interpreted as third window effects. Analysis of the OSL motion showed that this contribution results from a compressional motion of the OSL's vestibular and tympanic plates which is significantly lower in magnitude than the plates' translation in the direction of the stimulation. Separately applying OW input and rigid body stimulation provided insights into the interaction of BC sound entering via the OW and the reaction of the stapes to complex interior sound pressure distributions. Combined with the observations from a prior study (Kersten et al., 2024b) our results suggest a more important role for the OSL in BC hearing than previously understood. These findings enhance our understanding of the inner ear's response during BC and contribute to ongoing investigations into the interaction of BC mechanisms, while highlighting the need for further research into the deformation of the cochlear boundaries.

3D Computational Modeling of Blast Transmission through the Fluid-Filled Cochlea and Hair Cells

PURPOSE

Veterans commonly suffer from blast-induced hearing disabilities. Injury to the sensitive organ of Corti (OC) or hair cells within the cochlea can directly lead to hearing loss, but is very difficult to measure experimentally. Computational finite element (FE) models of the human ear have been used to predict blast wave transmission through the middle ear and cochlea, but these models lack a representation of the OC. This paper reports a recently developed 3D FE model of the OC to simulate the response of hair cells to blast waves and predict possible injury locations.

METHODS

Components of the OC model consist of the sensory cells, membranes, and supporting cells with endolymphatic fluid surrounding them inside the scala media. Displacement of the basilar membrane induced by a 31-kPa blast overpressure derived from the macroscale model of the human ear was applied as input to the OC model. The fluid-structure interaction coupled analysis in the time domain was conducted in ANSYS.

RESULTS

Major results derived from the FE model include the strains and displacements of the outer hair cells, stereociliary hair bundles (HBs), reticular lamina, and the tectorial membrane (TcM). The highest structural strain was concentrated around the connecting region of the HBs and the TcM, potentially indicating detachment due to blast exposure. Including the interstitial fluid in the OC created a realistic environment and improved the accuracy of the results compared to the previously published OC model without fluid.

CONCLUSION

The microscale model of OC was developed in order to simulate blast overpressure transmission through the fluid-filled cochlea and hair cells. This FE model represents a significant advancement in the study of blast wave transmission through the inner ear, and is an important step toward a comprehensive multi-scale model of the human ear that can predict blast-induced injury and hearing loss.

The Reduced Cortilymph Flow Path in the Short-Wave Region Allows Outer Hair Cells to Produce Focused Traveling-Wave Amplification

PURPOSE

Recent measurements show organ-of-Corti (OoC) motions that do not fit the classic hypothesis that outer hair cells (OHCs) amplify by pushing on the basilar membrane (BM) through stiff Deiters cells. One particularly surprising motion is that far below the best frequency (BF), the transverse motion of the OHC bottom is much greater than BM or reticular lamina (RL) motions.

METHODS

We explore this with (1) data from seven gerbils showing that the ratio, Rohc, of transverse motions at the OHC top to the OHC bottom is small at low frequencies but large near BF and (2) a heuristic model for the impedances of structures in a transverse cut through the OoC (the TOoC model) that accounts for Rohc.

RESULTS

The key idea is that when OHCs cyclically squeeze/expand, they force fluid out/into the space surrounding the OHCs which changes the local OoC area. At each time instant, cortilymph flows longitudinally along the tunnels from where OHCs squeeze to where OHCs expand, which is one-half the traveling-wave wavelength, λ. The impedance seen by OHCs for forcing cortilymph out/into and along the tunnels is termed ZOUT. Assuming that ZOUT decreases as λ gets shorter, the model Rohc shows the same frequency pattern as Rohc measurements.

CONCLUSION

Cyclic OHC forces produce OoC area changes consistent with those hypothesized to drive traveling-wave amplification. ZOUT variation with λ allows wide-band OHC motility to produce large OoC area changes and RL motions only near BF where λ is small, thereby producing narrow-band traveling-wave amplification. The model accounts for why, at low frequencies, the motion at the bottom of the OHCs is larger than BM motion. The model also explains why the OoC has longitudinal fluid spaces that connect to the fluid surrounding the OHCs.

Exploration of the dynamics of otic capsule and intracochlear pressure: Numerical insights with experimental validation

The otic capsule and surrounding temporal bone exhibit complex 3D motion influenced by frequency and location of the bone conduction stimulus. The resultant correlation with the intracochlear pressure is not sufficiently understood, thus is the focus of this study, both experimentally and numerically. Experiments were conducted on six temporal bones from three cadaver heads, with BC hearing aid stimulation applied at the mastoid and classical BAHA locations across 0.1-20 kHz. Three-dimensional motions were measured on various skull regions, including the promontory and stapes. Intracochlear pressure was measured using a custom acoustic receiver. The experiment was digitally recreated by a custom finite element model (FEM), based on the LiUHead, with the addition of an auditory periphery. The Young's modulus of the cortical bone domain within the FEM was varied between 4, 8, and 20 GPa. The predicted differential intracochlear pressures aligned with experimental data for most frequencies, and showed that skull deformation, particularly in the otic capsule, depends on skull material properties. Both experimental and FEM results indicated that the otic capsule behaves as a rigid accelerometer, imposing inertial loads on cochlear fluids, even above 7 kHz. Future work should explore the solid-fluid interactions between the otic capsule and cochlear contents.

Influence of the cochlear partition's flexibility on the macro mechanisms in the inner ear

Recent studies have highlighted the anatomy of the cochlear partition (CP), revealing insights into the flexible nature of the osseous spiral lamina (OSL) and the existence of a flexible cochlear partition bridge (CPB) between the OSL and the basilar membrane (BM). However, most existing inner ear models treat the OSL as a rigid structure and ignore the CPB, neglecting their potential impact on intracochlear sound pressure and motion of the BM. In this paper, we investigate the effect of the CP's flexibility by including the OSL and CPB as either rigid or flexible structures in a numerical anatomical model of the human inner ear. Our findings demonstrate that the flexibility of the OSL and the presence of the CPB significantly affect cochlear macro mechanisms, including differential intracochlear sound pressure, resistive behavior in cochlear impedances, CP stiffness, and BM velocity. These results emphasize the importance of considering the flexibility of the entire CP to enhance our understanding of cochlear function and to accurately interpret experimental data on inner ear mechanics.

Computational model of the human cochlea with motion of the layered osseous spiral lamina

Purpose

In the base of the human cochlea, the partition anatomy is distinct from the commonly recognized anatomy of laboratory animals. The human features a radially wide, osseous spiral lamina (OSL) and a soft-tissue bridge region that connects the OSL to the basilar membrane proper. In addition to the basilar membrane, the human OSL and bridge move considerably. We investigated the complex cochlear partition in human emphasizing the layered structure of the OSL with a finite element model. Model results were calibrated with experimental measurements of motion from optical coherence tomography.

Methods

The box model contained two fluid chambers separated by a cochlear partition and a helicotrema. Model geometrical and material properties either came from literature, measurements, or were tuned to produce a frequency-place map for the passive human cochlea as well as motion results similar to experimental measurements.

Results

The model motion results of the cochlear partition were similar to experimental results mostly within 5 dB but with differences at the high frequencies in both magnitude and phase beyond the best frequency. Around the best frequency location, the radial profile of cochlear partition motion was generally similar in both shape and magnitude. Sensitivity analysis, changing material-property parameters of the middle layer where the cochlear nerve fibers run between the layers of OSL plates, produced small changes in the model response and also showed negligible stress compared to the outer OSL plates.

Conclusion

These results suggest that the layered OSL anatomy is favorable as a conduit and protection for the nerve fibers while simultaneously functioning as a mechanical lever.

Microanatomy of the human tunnel of Corti structures and cochlear partition-tonotopic variations and transcellular signaling

Auditory sensitivity and frequency resolution depend on the optimal transfer of sound-induced vibrations from the basilar membrane (BM) to the inner hair cells (IHCs), the principal auditory receptors. There remains a paucity of information on how this is accomplished along the frequency range in the human cochlea. Most of the current knowledge is derived either from animal experiments or human tissue processed after death, offering limited structural preservation and optical resolution. In our study, we analyzed the cytoarchitecture of the human cochlear partition at different frequency locations using high-resolution microscopy of uniquely preserved normal human tissue. The results may have clinical implications and increase our understanding of how frequency-dependent acoustic vibrations are carried to human IHCs. A 1-micron-thick plastic-embedded section (mid-modiolar) from a normal human cochlea uniquely preserved at lateral skull base surgery was analyzed using light and transmission electron microscopy (LM, TEM). Frequency locations were estimated using synchrotron radiation phase-contrast imaging (SR-PCI). Archival human tissue prepared for scanning electron microscopy (SEM) and super-resolution structured illumination microscopy (SR-SIM) were also used and compared in this study. Microscopy demonstrated great variations in the dimension and architecture of the human cochlear partition along the frequency range. Pillar cell geometry was closely regulated and depended on the reticular lamina slope and tympanic lip angle. A type II collagen-expressing lamina extended medially from the tympanic lip under the inner sulcus, here named "accessory basilar membrane." It was linked to the tympanic lip and inner pillar foot, and it may contribute to the overall compliance of the cochlear partition. Based on the findings, we speculate on the remarkable microanatomic inflections and geometric relationships which relay different sound-induced vibrations to the IHCs, including their relevance for the evolution of human speech reception and electric stimulation with auditory implants. The inner pillar transcellular microtubule/actin system's role of directly converting vibration energy to the IHC cuticular plate and ciliary bundle is highlighted.

Quantitative Evaluation of the 3D Anatomy of the Human Osseous Spiral Lamina Using MicroCT

PURPOSE

The osseous spiral lamina (OSL) is an inner cochlear bony structure that projects from the modiolus from base to apex, separating the cochlear canal into the scala vestibuli and scala tympani. The porosity of the OSL has recently attracted the attention of scientists due to its potential impact on the overall sound transduction. The bony pillars between the vestibular and tympanic plates of the OSL are not always visible in conventional histopathological studies, so imaging of such structures is usually lacking or incomplete. With this pilot study, we aimed, for the first time, to anatomically demonstrate the OSL in great detail and in 3D.

METHODS

We measured width, thickness, and porosity of the human OSL by microCT using increasing nominal resolutions up to 2.5-µm voxel size. Additionally, 3D models of the individual plates at the basal and middle turns and the apex were created from the CT datasets.

RESULTS

We found a constant presence of porosity in both tympanic plate and vestibular plate from basal turn to the apex. The tympanic plate appears to be more porous than vestibular plate in the basal and middle turns, while it is less porous in the apex. Furthermore, the 3D reconstruction allowed the bony pillars that lie between the OSL plates to be observed in great detail.

CONCLUSION

By enhancing our comprehension of the OSL, we can advance our comprehension of hearing mechanisms and enhance the accuracy and effectiveness of cochlear models.

Intracochlear pressure and temporal bone motion interaction under bone conduction stimulation

BACKGROUND

Under bone conduction (BC) stimulation, the otic capsule, and surrounding temporal bone, undergoes a complex 3-dimentional (3D) motion that depends on the frequency, location and coupling of the stimulation. The correlation between the resultant intracochlear pressure difference across the cochlear partition and the 3D motion of the otic capsule is not yet known and is to be investigated.

METHODS

Experiments were conducted in 3 fresh frozen cadaver heads, individually on each temporal bone, resulting in a total of 6 samples. The skull bone was stimulated, via the actuator of a BC hearing aid (BCHA), in the frequency range of 0.1-20 kHz. Stimulation was applied at the ipsilateral mastoid and the classical BAHA location via a conventional transcutaneous (5-N steel headband) and percutaneous coupling, sequentially. Three-dimensional motions were measured across the lateral and medial (intracranial) surfaces of the skull, the ipsilateral temporal bone, the skull base, as well as the promontory and stapes. Each measurement consisted of 130-200 measurement points (∼5-10 mm pitch) across the measured skull surface. Additionally, intracochlear pressure in the scala tympani and scala vestibuli was measured via a custom-made intracochlear acoustic receiver.

RESULTS

While there were limited differences in the magnitude of the motion across the skull base, there were major differences in the deformation of different sections of the skull. Specifically, the bone near the otic capsule remained primarily rigid across all test frequency (above 10 kHz), in contrast to the skull base, which deformed above 1-2 kHz. Above 1 kHz, the ratio, between the differential intracochlear pressure and the promontory motion, was relatively independent of coupling and stimulation location. Similarly, the stimulation direction appears to have no influence on the cochlear response, above 1 kHz.

CONCLUSIONS

The area around the otic capsule appears rigid up to significantly higher frequencies than the rest of the skull surface, resulting in primarily inertial loading of the cochlear fluid. Further work should be focused at the investigation of the solid-fluid interaction between the bony walls of the otic capsule and the cochlear contents.

Models of Cochlea Used in Cochlear Implant Research: A Review

As the first clinically translated machine-neural interface, cochlear implants (CI) have demonstrated much success in providing hearing to those with severe to profound hearing loss. Despite their clinical effectiveness, key drawbacks such as hearing damage, partly from insertion forces that arise during implantation, and current spread, which limits focussing ability, prevent wider CI eligibility. In this review, we provide an overview of the anatomical and physical properties of the cochlea as a resource to aid the development of accurate models to improve future CI treatments. We highlight the advancements in the development of various physical, animal, tissue engineering, and computational models of the cochlea and the need for such models, challenges in their use, and a perspective on their future directions.

Otosclerosis under microCT: New insights into the disease and its anatomy

Purpose

Otospongiotic plaques can be seen on conventional computed tomography (CT) as focal lesions around the cochlea. However, the resolution remains insufficient to enable evaluation of intracochlear damage. MicroCT technology provides resolution at the single micron level, offering an exceptional amplified view of the otosclerotic cochlea. In this study, a non-decalcified otosclerotic cochlea was analyzed and reconstructed in three dimensions for the first time, using microCT technology. The pre-clinical relevance of this study is the demonstration of extensive pro-inflammatory buildup inside the cochlea which cannot be seen with conventional cone-beam CT (CBCT) investigation.

Materials and Methods

A radiological and a three-dimensional (3D) anatomical study of an otosclerotic cochlea using microCT technology is presented here for the first time. 3D-segmentation of the human cochlea was performed, providing an unprecedented view of the diseased area without the need for decalcification, sectioning, or staining.

Results

Using microCT at single micron resolution and geometric reconstructions, it was possible to visualize the disease's effects. These included intensive tissue remodeling and highly vascularized areas with dilated capillaries around the spongiotic foci seen on the pericochlear bone. The cochlea's architecture as a morphological correlate of the otosclerosis was also seen. With a sagittal cut of the 3D mesh, it was possible to visualize intense ossification of the cochlear apex, as well as the internal auditory canal, the modiolus, the spiral ligament, and a large cochleolith over the osseous spiral lamina. In addition, the oval and round windows showed intense fibrotic tissue formation and spongiotic bone with increased vascularization. Given the recently described importance of the osseous spiral lamina in hearing mechanics and that, clinically, one of the signs of otosclerosis is the Carhart notch observed on the audiogram, a tonotopic map using the osseous spiral lamina as region of interest is presented. An additional quantitative study of the porosity and width of the osseous spiral lamina is reported.

Conclusion

In this study, structural anatomical alterations of the otosclerotic cochlea were visualized in 3D for the first time. MicroCT suggested that even though the disease may not appear to be advanced in standard clinical CT scans, intense tissue remodeling is already ongoing inside the cochlea. That knowledge will have a great impact on further treatment of patients presenting with sensorineural hearing loss.